Posts Tagged interface

External Preloader; more complex cases

In my previous post I spoke about what would be my optimal external preloader, but I really covered only simple examples. In relations to external preloaders, there are 2 cases where it gets more complex. First, when you want to time you preloader’s animation with your main content’s animation and second, when you use FlashVars (variables passed from the HMTL to the SWF).

Timing a preloader animation with the main content animation

Sometimes you don’t want to start the loaded content as soon as it’s been loaded.  You want to leave time for the preloader’s exit animation to end or you want to morph your preloader with the main content to have some sort of continuity. In those cases, the external preloader has to call a method of the main content. That sounds simple but it actually isn’t. If you try to call a method directly like we used to do in ActionScript 2 the compiler will complain because it doesn’t know if the loaded content will have such a method. In order to solve this problem you will have to use interfaces. You can get more information about interfaces on this previous post of mine. Now that the loaded content is typed to an interface you will be able to call it’s method and the synchronization between the preloader and the content will be good.

Using FlashVars

FlashVars are really useful and a lot of times you might end up using them. The problem is that contrarily to ActionScript 2, in AS3 the root variable will refer to this SWF’s root, not to the entire application’s root. So from the loaded content you don’t have access to the FlashVars. You solve this problem the same way as we solved the preceding problem: by using an interface. In the first case, we mostly would have called a method without parameters like init() or something like that, but in this case the parameters will be the FlashVars. You might want to pass the root.loaderInfo.parameters.YOUR_VAR_NAME as a parameter or you might want to do some checking as to know if the parameter was passed or not. In your interface you will have to write down all the parameters you want to pass and what type (String, int, Number, etc) they are. Here is an example of how you could do it.

Here is the interface:

package com.zedia.interfaces{
  public interface IMain{
    function init(flashvars1:String, flashvars2:Number):void;

Here is some of the code of that the preloader could contain:

import com.zedia.interfaces.IMain;
var mainContent:IMain;
function onLoadComplete(event:Event):void{ // this would be the function that the loader would call when the loading is completed
  mainContent = IMain(loader.content);
  addChild(Sprite(mainContent) );
  mainContent.init( String(root.loaderInfo.parameters.flashvars1),Number(root.loaderInfo.parameters.flashvars2) )

Here is some code of the loaded content:

  import com.zedia.interfaces.IMain;
  public class Main extends Sprite implements IMain{
    public function init(flashvars1:String, flashvars2:Number):void{
      //do something with the FlashVars

As you can see in the preloader’s ActionScript, in order to add the content to the stage, you first have to cast it to Sprite. You will have to do this when you will want to have access to Sprite properties like x and y. I admit this is a down side of using interfaces but at the same time it solved both the problems of timing an animation and passing FlashVars to the loaded content.

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How to use interface in ActionScript 3

There are a lot of good ActionScript 3 books out there, but in most of them all the examples about inheritance, composition, interface, polymorphism and more complex object oriented principles are done using data classes. It’s good to explain the principles, but when you are ready to code some actual classes in Flash, some problems start to arise. Most of my problems come from the fact that 90% of the time I am coding classes that are a visual representation of something. So these classes extends the Sprite or MovieClip class.

So I was coding this game class where I have multiple kind of monsters but I want to use the same functions without checking what kind of monster it is. This is a case where polymorphism comes in handy. The thing is I didn’t do my classes of monster using inheritance (which maybe I should have done), so in order to use polymorphism, I have to create an interface with the public methods common to all my monster classes. So I open the Colin Moock book and start looking at how to code interfaces. Here is a list of quick facts that will help you while doing so.

  • You have to list all public methods that will be common for the classes that implements this interface;
  • You do so by writing the definition of the function, its name, its parameters and its return type;
  • You don’t have to open and close curly braces after the method definition
  • You don’t specify if the method is private, public or protected (that’s kind of obvious but I did that mistake)
  • interface can only extend other interfaces
  • a class can implement more than one interface

So here is an example of an interface in As3:

  public interface IMonster {
    function getShot(damage:uint):void;
    function getCanShoot():Boolean;

Here is how my visual class implements this interface while also extending the MovieClip class:

public class spriteMouton extends MovieClip implements IMonster{

It is important that you put the extend keyword before the implement keyword.

So most of what I just wrote about was details about interfaces, but when you start coding, all these details are important to know.

Here are two more article I wrote about interfaces:

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